1. Field of the Invention
The present invention relates to a method of manufacturing an aluminum alloy pipe of high strength, and in particular to a method suitable for manufacturing a front-fork outer tube for a two-wheeled vehicle.
2. The Related Art of the Invention
Japanese Unexamined Utility Model Publication No. 2-143293 has disclosed as one method of manufacturing a front-fork outer tube for a two-wheeled vehicle using an aluminum alloy pipe of high strength such as an Al—Zn—Mg family aluminum alloy or an Al—Zn—Mg—Cu family aluminum alloy a method of cutting an aluminum pipe of high strength after age-hardening.
FIG. 10 shows the processes of this manufacturing method. A pipe according to the manufacturing method is manufactured by the order of the following processes:                (1) The process of performing solution treatment by heating a pipe (referred to as solution process 1 hereinafter as needed),        (2) The process of performing mill treatment by milling the pipe (referred to as mill process 2 hereinafter as needed),        (3) The process of performing artificial aging treatment by reheating the pipe (referred to as artificial aging process 3 hereinafter as needed), and        (4) The process of performing cutting treatment necessary for the pipe (referred to as cutting treatment 4 hereinafter as needed).        
And there is another method of manufacturing a pipe by the processes shown in FIG. 11. Namely a pipe is manufactured by the order of the following processes.                (1) The process of performing softening treatment by heating a pipe to a temperature within a certain range (referred to as softening process 5 as needed),        (2) The process of performing plastic working treatment to the pipe (referred to as plastic working process 6 hereinafter as needed),        (3) Solution process 1 of performing solution treatment 1.        (4) Artificial treatment process 3 of performing artificial aging treatment, and        (5) Cutting process 4 of performing cutting work treatment.        Forming the outer tube of the front fork for a motor bike by the above-described methods allows light in weight of an entirety of a pipe material and establishment of strength of a part of the pipe material.        